Thursday, March 26, 2009

If your new to Cocoa you've probably not heard of Carbon, on the other hand if you've been developing for the Mac for a while now you've probably at least heard of Carbon and if you're really experienced you've probably used carbon. If you've used carbon at all this article really isn't for you. What I want to do is gently introduce you to carbon events, the 1 scenario where we'd use it for in terms of cocoa apps, why Cocoa Events can't handle this one scenario and why Carbon Events can, and how to implement it in a Cocoa Application.
Why Carbon Events
There is a specific scenario I had in mind as to why you would use Carbon Events ( actually if you have a Cocoa app it's a you have to use Carbon Events), the scenario is this: you have a Cocoa application and you know your customers will be running it but it will not be the active application running in the foreground, your customers may be looking at Mail, NetNewsWire, Xcode, etc basically some other application than yours and yet you still want to receive a keyboard shortcut key pressed event. How do you do this? Not with Cocoa, in its current form it's literally impossible. Here's why
Cocoa Events works as such, according to the Cocoa Event Handling Guide it says "In the main event loop, the application object (NSApp) continuously gets the next (topmost) event in the event queue, converts it to an NSEvent object, and dispatches it toward its final destination. It performs this fetching of events by invoking the nextEventMatchingMask:untilDate:inMode:dequeue: method in a closed loop. When there are no events in the event queue, this method blocks, resuming only when there are more events to process."
It then goes on "After fetching and converting an event, NSApp performs the first stage of event dispatching in the sendEvent: method. In most cases NSApp merely forwards the event to the window in which the user action occurred by invoking the sendEvent: method of that NSWindow object. The window object then dispatches most events to the NSView object associated with the user action in an NSResponder message such as mouseDown: or keyDown:. An event message includes as its sole argument an NSEvent object describing the event."

So in other words the NSApplication has an event queue and receives an event (1) converts it into NSEvent objects (2) which it then forwards to a Window (3) which then dispatches the event to the view associated with the user action calling on it -mouseDown:(NSEvent *)event, -keyDown:(NSEvent *)event, etc (4).
This is great, but if our application is running in the background there is no way our Window will receive events. Cocoa Events simply does not allow for a regular cocoa application to be in the background and receive keyboard events. Now I could go on to further explain NSResponder and how it events could then go onto the menu's after exhausting all possible views in a window that could respond to an event, but that's another article onto itself. The point is our app is a regular cocoa app and it has a window and menu's, but we know our application is not going to have focus and thus won't have a window that can forward events or even application menu's to receive events, but we still want to receive an event and invoke some sort of functionality.
There are plenty of examples of this: OmniFocus has a quick entry window which you can invoke from anywhere, applications like Quicksilver use Carbon Events so you can invoke them anywhere from in Mac OS X. These are both Cocoa applications, but they depend on Carbon Events to give them notifications of global keyboard shortcut events. Conceptually Carbon events is vastly different from Cocoa Events. When I first finally got a Carbon Events example working it seemed very strange compared to Cocoa Events, it was more like there is a stream of events going by in the system and you install something to indicate a interest in a particular kind of event. If that event matches the specific event you are looking for then your code is called and you have a global keyboard shortcut. So how do we do this?
Setting up a project
In Xcode, go ahead and create a new Cocoa application. At this point if you build in go all you will have is a small window and an application that does nothing. Now go to File->New File and create a plain Cocoa class and call it AppController. Then click on the disclosure triangle by the Frameworks folder and then right-click on Linked Frameworks and go to Add -> Existing Frameworks and in your SDK add "Carbon.Framework."
At this point this should be all you have for AppController.h

#import <Cocoa/Cocoa.h>
@interface AppController : NSObject {
}
@end

Now in AppController.m add
#import <Carbon/Carbon.h>
and implement -(void)awakeFromNib
and you should have

this method will be the rough equivalent of implementing say -(void)mouseDown:(NSEvent *)event in a cocoa class. The Carbon Event handler will pass the EventHandlerCallRef (next event handler reference), EventRef (roughly equivalent to NSEvent) arguments to us. If we were doing really deep Carbon Events work then we might also be interested in all the arguments, but for this simplistic task all we need is the EventRef reference.
Into the guts of Carbon Events
Now we've set the foundation so now lets go to work in the awake from Nib method. Add the following code to the top of the awakeFromNib Method

Now lets understand these new variables.
EventHotKeyRef : The documentation is pretty descriptive about this "Represents a registered global hot key." In essence we have this reference so we can unregister it later if we want to.
EventHotKeyID : Again "Represents the ID of a global hot key." This is where you'll set some things to uniquely identify your global keyboard shortcut.
EventTypeSpec : "Describes the class and kind of an event." Passing in this reference after setting the class and kind of event tells the event handler what type of event this is.
So lets set the EventTypeSpec reference

this installs our event handler method we declared earlier, but haven't implemented yet, so when this event happens it will call our code. Now we need to uniquely identify our hotKey...

myHotKeyID.signature='mhk1';
myHotKeyID.id=1;

If you just intend on having only 1 global keyboard shortcut then this doesn't matter too much, but if you intend on having multiple global keyboard shortcuts then the id will matter greatly, it's what you will use to uniquely identify each specific event in the method we will implement shortly.

This is what actually registers our global shortcut. In Essence it's arguments are (1) the key code to the key you will use in combination with (2) modifier keys to create your global keyboard shortcut and (3) the Application Event Target which the GetApplicationEventTarget() method retrieves for us and finally (4) the EventHotKeyRef we declared earlier which gives us a reference to this keyboard shortcut we are registering. In this case it registers the spacebar (int 49) plus the command key and option key. So when Command+Option+Spacebar is hit our global keyboard shortcut will be triggered.
It's important to note that you cannot register the same key combination twice within the same application and have it trigger both methods, however multiple applications can register for the same global keyboard shortcut and Carbon events will trigger events in both applications. I decided to test this by duplicating the keyboard shortcut I use for OmniFocus quick entry which for me is Control+Option+Space Bar

Well then you have to use GetEventParameter() as such to get the EventHotKeyID which you can then get it's ID code and handle the event appropriately. This would also involve creating a 2nd hot key and setting it's id to 2, 3,etc..
Key Code
The only thing left is the integer key code I mentioned earlier. I talked to Eric Rocasecca and Jim Turner from Startly (who make QuicKeys which I think would use Carbon Events, but evidently they use their own system) about this and they had a magazine with a reference of all the key codes in it, I didn't have this and didn't want to bother hunting down an old rare magazine. Several Google searches later I found an old app called AsyncKeys! which tells you the integer code of any key you press. I downloaded it a long time ago, but I did turn up a couple locations that actually had a download as the original site appears to be long gone (really sorry but I can only find .sit files): http://asynckeys.mac.findmysoft.com/ and if you Google around you can find other download locations.
Here are some random key codes
Spacebar 49
a = 0
1=18
F1 = 122
Left arrow = 123
Down arrow = 125
Right arrow = 124
Up Arrow = 126
Enter = 36
Backspace = 51
` = 50
and I could go on and on...
Conclusion
Essentially we saw a brief intro into how Cocoa Events work, saw how carbon events work, installed an event handler in our application, registered a global hot key and saw how to handle multiple global hot keys. This is a narrow use scenario, not many apps will need to use this, but if you want to activate a particular function and your application is Cocoa and running in the background, this is the best way you achieve this functionality. I really wouldn't recommend digging into Carbon more as Apple is moving to deprecate it and as a Cocoa developer you should be finding that you'll need carbon less and less as time goes on.

Eimantas: I would credit him, but I have only seen that recently in the past couple weeks. I first learned about all of this actually by looking at Quicksilver's source code where I first saw InstallEventHandler() used and then started researching from there.

I want to use hotkey with a command line service code using mac. i try to use this code in my command line service but it is not working there. can u help how we use this with command line service code?